I'm currently using gendoc with ReqIFStudio to generate my documentation out of an ReqIF format.
doing that I would like to change the orientation of some section of my document for portrait to landscape but I don't know if that's actually feasible with gendoc

Hello
i don't know if other solutions exist but in previous experience we did something like that
[if (someCondition)]
_____________________________________________________________
|
| page set as landscape
|
|____________________________________________________________

Actually the difficulty is more about installing gendoc on a special version of eclipse as may dependencies cannot be satisfied. About browsing the pain point is when the value of the item is in XHTML then i geuss a new model should be used .
today I've simply used as a workaround to use only string rather than enriched string

OK what are the issues you have with older version of Eclipse,
just for information in my former company where we started the development of Gendoc thanks to the commercial support we could develop some adapted packages for older versions of Eclipse